1. Field of the Invention
The present invention relates to an interlayer dielectric film, a semiconductor device and a solid-state image pickup device using the same, and a method of manufacturing the same.
2. Description Of The Related Prior Art
The structure of a conventional semiconductor device is described below while referring to FIG. 9 as an example of sectional view of a solid-state image pickup device.
Impurities are selectively implanted in an N type semiconductor substrate 1, and a photodiode 2 and a transfer channel 3 are formed by thermal diffusion. The photodiode 2 generates photo carriers by photoelectrically converting the light entering from outside in its inside. On the transfer channel 3, a transfer gate electrode 5 is formed on the N type semiconductor substrate 1 through a gate dielectric film 4. When a specific voltage pulse is applied to the transfer gate electrode 5, the photo carries in the photodiode 2 are transferred to the transfer channel 3. An interlayer file 6 is formed so as to cover the transfer gate electrode 5. A first light-shield film 7 formed in a specific region on the interlayer film 6 is formed so as to shield the light from entering into the transfer channel 3. As a result, the smear noise characteristic of the solid-state image pickup device is improved, as reported in the Japanese Laid-open Patent Hei. 2-156670.
In the portion contacting with the N type semiconductor substrate 1, a contact hole 9 is formed by removing the gate dielectric film 4, interlayer film 6 and interlayer dielectric film 8. In the bottom of the contact hole 9, a diffusion layer 10 formed on the N type semiconductor substrate 1 is exposed. Through this contact hole 9, a wiring layer 11 is connected to the diffusion layer 10. The diffusion layer 10 is formed in order to connect the N type semiconductor substrate 1 and the wiring layer 11 well electrically. A second light-shield film 12 is formed on the first light-shield film 7 through the interlayer dielectric layer 8. Thus, the smear noise of the solid-state image pickup device may be reduced.
Furthermore, a final protective film 13 is provided in order to protect the uppermost surface of the solid-state image pickup device.
Referring next to FIG. 10, a conventional method for manufacturing a solid-state image pickup device is described below.
In FIG. 10 (a), an N type silicon substrate is used as the N type semiconductor substrate 1. In the N type semiconductor substrate 1, a photodiode 2 and a transfer channel 3 are formed by ion implantation and thermal diffusion. Afterwards, on the N type semiconductor substrate 1, a gate dielectric film 4, a transfer gate electrode 5, an interlayer film, 6, a first light-shield film 7, and a second interlayer dielectric film 8 are formed sequentially (FIG. 10 (b)).
Consequently, in nitrogen (N.sub.2) atmosphere, flattening heat treatment is conducted at 900.degree. C. or higher. As a result of the heat treatment, the interlayer dielectric film 8 is flattened. Hence, in the convex part of the matrix, the film thickness of the interlayer dielectric film 8 decreases. For example, in the shoulder part A of the step where the first light-shield film 7 is formed, the film thickness of the interlayer dielectric film 8 is extremely thin (FIG. 10 (c)).
Next, a contact hole 9 is opened by plasma etching. Then, in the gas containing phosphorus in the composition, for example, in the atmosphere of phosphine (PH.sub.3) or the like, heat treatment is conducted at 900.degree. C. or higher, and a phosphorus glass film (P.sub.2 O.sub.5 film) (not shown) is formed on the entire surface. At the same time, from this phosphorus glass film, phosphorus is diffused thermally into the N type semiconductor substrate 1. In this way, the N type diffusion layer is formed as a diffusion layer 10.
Subsequently, using a chemical solution of water and hydrofluoric acid mixed at a rate of 20:1, the phosphorus glass film is removed by wet etching. In this wet etching, the chemical solution is hardly circulated in the bottom of the fine contact hole. Hence, sufficiently overetching is done so that the phosphorus glass film may not be left over. when using the chemical solution, empirically, the etching time of about 20 seconds is required. In the region where the circulation of chemical solution is promoted smoothly, the phosphorus glass film is removed promptly, so that the interlayer dielectric film 8 is also etched (FIG. 10 (d)).
Next, by sputtering method, an aluminum film is deposited on the entire substrate surface in a film thickness of 1 .mu.m. Then, using the resist pattern (not shown) as the mask, plasma etching is effected to form a wiring layer 11 and a second light-shield film 12. Lastly, on the uppermost surface of the device, a silicon oxide film (SiO.sub.2 film) is formed by plasma enhanced CVD method as a final protective film 13.
In such prior art, however,if the interlayer dielectric film 8 is formed thickly, the aspect ratio of the contact hole 9 formed therein becomes larger. Hence, it is difficult to etch adequately. Besides, because of the large aspect ratio, a step is formed in the wiring layer 11 on the contact hole 9. The same phenomenon occurs not only in the solid-state image pickup device, but also in other semiconductor devices.
As an intrinsic problem of the solid-state image pickup device, moreover, if the interlayer dielectric film 8 is formed thickly, the light enters the first light-shield film 7 from an oblique direction of the interlayer dielectric film 8. Since the first light-shield film 7 is formed in a proper thickness in consideration of the processability, part of the light penetrates through the first light-shield film 7, thereby deteriorating the smear noise characteristic of the CCD solid state image pickup device.
However, when it is attempted to apply the structure of the conventional semiconductor device and its manufacturing method in the solid-state image pickup device, in order to obtain a sufficient breakdown voltage of the interlayer dielectric film 8, the interlayer dielectric film 8 must be formed thickly beforehand, in consideration of the decrease of film thickness in the convex part of the matrix caused by flattening process.
Yet, generally, wet etching is large in the fluctuations of etching rate, and hence a large variance occurs in the decreasing the film thickness of the interlayer dielectric film 8 by overetching at the time of removal of the phosphorus glass film. Empirically, the film thickness of the interlayer dielectric film 8 varies in a range of about 20 to 40 nm in the wafer, between wafers, and between lots. Due to such fluctuations of the film thickness and necessity for obtaining specific interlayer dielectric breakdown voltage, it is more and more difficult to form the interlayer dielectric film 8 thinly.
The invention relates to the semiconductor device solving such problems of difficulty in thinning of the interlayer dielectric film.